KR20130014068A - High-tension/hot-rolled steel sheet having excellent workability, and method for producing same - Google Patents

Abstract

Provided are a high tensile strength hot rolled steel sheet having strength and workability (elongation flangeability) and a method of manufacturing the same.
In mass%,
C: 0.005% or more, 0.050% or less, Si: 0.2% or less,
Mn: 0.8% or less, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less
Is contained so that S, N, and Ti satisfy Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48), and the balance is made of Fe and inevitable impurities, and the area ratio of the entire ferrite structure And 95% or more of the matrix, and fine carbide containing Ti and having an average particle diameter of less than 10 nm are dispersed and precipitated. This excellent high strength hot rolled steel sheet is obtained.

Description

High-strength hot rolled steel sheet with excellent workability and manufacturing method {HIGH-TENSION / HOT-ROLLED STEEL SHEET HAVING EXCELLENT WORKABILITY, AND METHOD FOR PRODUCING SAME}

TECHNICAL FIELD The present invention relates to a high tensile strength hot rolled steel sheet excellent in workability (elongation flange property), which is suitable for transport substrates such as automobile parts and structural materials.

In order to reduce CO ₂ emissions from the viewpoint of global environmental conservation, it is always an important task for the automobile industry to maintain the strength of the automobile body and to reduce its weight and to improve the fuel economy of the automobile. In order to reduce the weight of the vehicle body while maintaining the strength of the automobile body, it is effective to thin the steel sheet by increasing the strength of the steel sheet serving as the material for automobile parts. For example, high-strength thinning of steel sheets for suspension parts of automobiles is a very effective means for improving automobile fuel efficiency, because it leads to a significant reduction in weight of automobile bodies. For this reason, the demand for high strength for these component materials is very strong.

On the other hand, since many automotive parts made of steel sheet are formed by press working, burring, or the like, it is required to have excellent elongation and elongation flangeability in the steel sheet for automotive parts. For example, since a suspension part has a complicated shape, workability is important to a steel plate as a material for suspension parts, and a high tensile strength steel plate excellent in workability such as elongation and elongation flangeability is required.

In general, however, steel materials are deteriorated in workability with increasing strength. Therefore, in applying a high tensile strength hot rolled sheet steel to suspension parts etc., development of the high tensile strength hot rolled steel sheet which combines strength and workability becomes essential, and until now, many researches are made and various techniques are proposed.

For example, in Patent Literature 1, in weight%, C: 0.03 to 0.25%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.1% or less, S: 0.007% or less, Al: 0.07% or less and Cr: It is a composition containing 1.0% or less, and it is set as the composite structure which consists of ferrite and a 2nd phase (1 or more types of pearlite, bainite, martensite, and retained austenite), and the hardness, volume fraction, And by defining the particle diameter, the technique of improving the fatigue characteristic and extension | stretching flange property of the high strength hot rolled sheet steel whose tensile strength (TS) exceeds 490 N / mm <2> (490 Mpa) is proposed.

Moreover, in patent document 2, in wt%, C: 0.01 to 0.10%, Si: 1.5% or less, Mn: more than 1.0%-2.5%, P: 0.15% or less, S: 0.008% or less, Al: 0.01 to 0.08 %, Ti, Nb, or a total of one or two kinds: chemical composition containing 0.10 to 0.60%, the amount of ferrite is 95% or more by area ratio, and the average grain size of ferrite is 2.0 to 10.0 µm, and martensite By setting it as a structure which does not contain a ziite and residual austenite, the technique of improving the fatigue strength of a high strength hot rolled sheet steel whose tensile strength (TS) is 490 Mpa or more, especially an extension flange property is proposed. And in the technique proposed by patent document 2, when the Mn content is made into more than 1.0%-2.5%, while steel plate strength improves, fine ferrite particle is obtained.

Moreover, in patent document 3, in mass%, C: 0.01 to 0.1%, S <0.03%, N <0.005%, Ti: 0.05 to 0.5%, Si: 0.01 to 2%, Mn: 0.05 to 2%, P ≤0.1%, Al: 0.005-1.0%, and further contains Ti in a range satisfying Ti-48 / 12C-48 / 14N-48 / 32S≥0%. 10, the average size of the precipitates containing ㎚ or more Ti ¹ ~ 10 ³ ㎚ a minimum distance to 10. ¹ ㎚ than 10 ⁴ by a ㎚ or less, tensile strength of burring workability and fatigue characteristics of high strength hot rolled steel sheet not less than (TS) is 640 ㎫ A technique for improving the performance has been proposed.

Japanese Patent Laid-Open No. 4-329848 Japanese Laid-Open Patent Publication 2000-328186 Japanese Unexamined Patent Publication No. 2002-161340

However, in the technique proposed in Patent Literature 1, when forming a desired part shape by pressing a steel sheet or the like, the interface between the soft ferrite and the hard second phase tends to be a starting point of cracking during processing. It has a problem that it is not stable. Moreover, in the technique proposed by patent document 1, when the tensile strength (TS) of a steel plate is raised to 590 Mpa class, the problem that workability, especially elongation flange property, is inadequate for the requirement of the present situation can also be seen (patent document 1). See examples of).

In addition, in the technique proposed in Patent Document 2, since the Mn content of the steel sheet is high, Mn segregates in the center of the plate thickness of the steel sheet, and causes cracks during processing during press molding of the steel sheet, thus providing excellent elongation flangeability. This is difficult to secure, and it is not always possible to obtain sufficient elongation flangeability. In addition, in the technique proposed in Patent Literature 2, Ti carbide is formed with a predetermined content to reduce the solid solution C which adversely affects the elongation flangeability. Carbide tends to coarsen and the problem is that the desired strength cannot be obtained stably.

Moreover, in the technique proposed by patent document 3, the distribution of the size of the precipitate contained in a steel plate is large, and the problem that a desired strength cannot be stably ensured is seen. Moreover, in the technique proposed by patent document 3, the elongation flange property of a steel plate is inadequate (refer Example of patent document 3).

For automobile parts to be mass-produced, it is necessary to industrially mass-produce hot-rolled steel sheets in order to stably supply the material. In the above-described prior art, the tensile strength (TS) is 590 MPa or more and excellent workability (extension plan) It is difficult to stably supply a high tension hot rolled steel sheet having oiliness). The present invention advantageously solves the problems of the prior art described above, and has a tensile strength (TS) of 590 MPa or more, which is preferable as a material for automobile parts, and is excellent in workability (extension flange property), specifically, hole expansion. Rate (lambda): It aims at providing the high tension hot rolled sheet steel which has 100% or more, and its manufacturing method.

MEANS TO SOLVE THE PROBLEM In order to solve the said subject, the present inventors earnestly examined the various factors which affect the high strength of a hot rolled sheet steel and workability (extension flange property). As a result, the following findings were obtained.

1) When the steel sheet structure is made into a ferrite single phase structure excellent in workability with low dislocation density, and the fine carbide is dispersed and precipitated, the strength is improved while maintaining the elongation flangeability of the hot rolled steel sheet.

2) In order to obtain a hot rolled steel sheet excellent in workability and having a high strength of tensile strength (TS) of 590 MPa or more, it is necessary to disperse and precipitate fine carbide having an average particle diameter of less than 10 nm effective for precipitation strengthening at a desired volume ratio.

3) As a fine carbide which contributes to precipitation strengthening, a carbide containing Ti is effective from the viewpoint of securing strength.

4) In order to make carbide containing Ti an average particle diameter: less than 10 nm and to disperse precipitate at a desired volume ratio, it is necessary to secure the amount of Ti to form Ti carbide as a precipitation nucleus, and N in steel as a material. It is necessary to contain Ti (Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48)) of a predetermined amount or more with respect to the S content.

5) In depositing carbide containing Ti in the ferrite phase finely (average particle size: less than 10 nm), the B content and the Mn content in the steel serving as the raw material are controlled at a desired ratio (B≥0.001 × Mn). What is valid to do.

6) When Ti content of the carbide containing Ti exceeds C content by an atomic ratio, a carbide will become coarse easily and will have a bad influence on a hot rolled sheet steel characteristic.

7) In order to suppress the coarsening of carbides by setting the Ti content of the carbide containing Ti to less than the C content in an atomic ratio, the Ti, N, and S contents with respect to the C content in the steel used as the raw material are determined in a predetermined range ( C / 12> Ti / 48-N / 14-S / 32) is effective.

This invention is completed based on said knowledge, The summary is as follows.

[1] at mass%

C: 0.005% or more, 0.050% or less, Si: 0.2% or less,

Mn: 0.8% or less, P: 0.025% or less,

S: 0.01% or less, N: 0.01% or less,

Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less

Containing S, N, and Ti so as to satisfy the following formula (1), the balance comprising Fe and inevitable impurities, a matrix having an area ratio of 95% or more with respect to the entire ferrite structure, and Ti A fine carbide having an average particle diameter of less than 10 nm is dispersed and precipitated, and has a structure having a volume ratio of 0.0007 or more to the entire structure of the fine carbide, and has a tensile strength of 590 MPa or more.

Ti? 0.04 + (N / 14 x 48 + S / 32 x 48)... (One)

(S, N, Ti: content of each element (mass%))

[2] The high-strength hot rolled steel sheet having excellent workability according to the above [1], further comprising B: 0.0003% or more and 0.0035% or less by mass% so as to satisfy the following formula (2) in addition to the above composition. .

B ≧ 0.001 × Mn... (2)

(Mn, B: content of each element (mass%))

[3] The high tensile strength hot rolled steel sheet according to the above [1] or [2], wherein the composition satisfies the following formula (4).

C / 12> Ti / 48-N / 14-S / 32... (4)

(C, S, N, Ti: content of each element (mass%))

[4] The high-strength hot rolled sheet having excellent workability according to any one of [1] to [3], wherein the fine carbide is a carbide satisfying the following formula (3) with an atomic ratio of C and Ti to be contained. Grater.

C / Ti> 1.0... (3)

(C / Ti: Atomic Ratio of C and Ti in Fine Carbide)

[5] In any one of [1] to [4], in addition to the above composition, furthermore, in mass%, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, High tensile strength hot rolled steel sheet excellent in workability which contains 0.1% or less in total of any 1 or more types of Ta.

[6] The high tensile strength hot rolled steel sheet according to any one of [1] to [5], having a plated film on the steel sheet surface.

[7] When the steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, it is cooled and wound to form a hot rolled steel sheet.

The steel material in mass%,

C: 0.005% or more, 0.050% or less, Si: 0.2% or less,

Mn: 0.8% or less, P: 0.025% or less,

S: 0.01% or less, N: 0.01% or less,

Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less

Is contained so that S, N, and Ti satisfy the following formula (1), and the balance is made of Fe and unavoidable impurities.

The finish rolling temperature of the said finish rolling is 880 degreeC or more, the average cooling rate of the said cooling is 10 degreeC / s or more, and the winding temperature is 550 degreeC or more and less than 800 degreeC, The tensile strength is 590 Mpa The manufacturing method of the high tension hot rolled sheet steel excellent in the above workability.

Ti? 0.04 + (N / 14 x 48 + S / 32 x 48)... (One)

(S, N, Ti: content of each element (mass%))

[8] The high-strength hot rolled steel sheet having excellent workability according to the above [7], further comprising B: 0.0003% or more and 0.0035% or less by mass% to satisfy the following formula (2) in addition to the above composition. Method of preparation.

B ≧ 0.001 × Mn... (2)

(Mn, B: content of each element (mass%))

[9] The method for producing a high tensile strength hot rolled steel sheet according to the above [7] or [8], wherein the composition satisfies the following formula (4).

C / 12> Ti / 48-N / 14-S / 32... (4)

(C, S, N, Ti: content of each element (mass%))

[10] In any one of [7] to [9], in addition to the above composition, further, in mass%, Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, Any one or more types of Ta are contained 0.1% or less in total, The manufacturing method of the high tension hot rolled sheet steel excellent in workability.

According to the present invention, high-strength hot rolled steel having excellent workability (elongation flange property), which is suitable for a steel sheet for automobiles and the like, and is suitable for materials such as suspension parts having a tensile strength (TS) of 590 MPa or more and a complicated cross-sectional shape at the time of pressing. It becomes possible to provide a steel plate, and exhibits the outstanding effect industrially.

Hereinafter, the present invention will be described in detail.

First, the structure of the steel sheet of the present invention and the reason for limitation of carbide will be described.

In the hot-rolled steel sheet of the present invention, a matrix in which the ferritic phase is 95% or more by area ratio with respect to the entire structure, and fine carbide containing Ti in the matrix and having an average particle diameter of less than 10 nm are dispersed and precipitated, and the fine carbide is applied to the whole structure. It has a tissue whose volume ratio is 0.0007 or more.

Ferrite phase: 95% or more in area ratio of the entire structure

In this invention, formation of a ferrite phase becomes essential in ensuring the workability (elongation flange property) of a hot rolled sheet steel. It is effective to make the structure of a hot rolled steel sheet into the ferrite phase which is excellent in ductility with low dislocation density for improvement of elongation of a hot rolled steel sheet, and extension | stretching flange property. In particular, it is preferable to make the structure of the hot-rolled steel sheet a ferrite single-phase structure to improve the elongation flange property, but even when it is not a complete ferrite single-phase structure, the ferrite single-phase structure, that is, 95% or more in terms of the area ratio of the entire structure. If it is a ferrite phase, the said effect will fully be exhibited. Therefore, the area ratio with respect to the whole structure of a ferrite phase is made into 95% or more. Preferably, it is 97% or more.

In addition, in the hot-rolled steel sheet of the present invention, examples of structures other than ferrite phase include cementite, pearlite, bainite phase, martensite phase, residual austenite phase and the like, and the sum of these sums is the area ratio of the entire structure. Furnace is about 5% or less, preferably about 3% or less is acceptable.

Fine carbide containing Ti

Carbide containing Ti tends to be a fine carbide having a very small average particle diameter. Therefore, in this invention which aims at high strength of a hot rolled sheet steel by disperse | distributing and depositing fine carbide in a hot rolled sheet steel, it is set as the fine carbide containing Ti as a fine carbide to carry out dispersion precipitation. Moreover, as mentioned later, in this invention, it is preferable to set it as fine carbide so that the atomic ratio of C and Ti in fine carbide may satisfy | fill predetermined conditions.

Average particle size of fine carbide: Less than 10 nm

In providing the desired strength (tensile strength: 590 MPa or more) to the hot-rolled steel sheet, the average particle diameter of the fine carbides is very important. In the present invention, the average particle diameter of the fine carbide containing Ti is made less than 10 nm. When the fine carbide precipitates in the matrix, the hot carbide is strengthened by acting as a resistance to the shift of dislocation generated when deformation is applied to the steel sheet, but when the average particle diameter of the fine carbide is less than 10 nm, Becomes more pronounced. Therefore, the average particle diameter of the fine carbide containing Ti is less than 10 nm. More preferably 5 nm or less.

Volume ratio with respect to the whole structure of fine carbide: 0.0007 or more

In providing the desired strength (tensile strength: 590 MPa or more) to the hot-rolled steel sheet, the dispersed precipitation state of the fine carbide containing Ti is also very important. In the present invention, fine containing Ti and having an average particle diameter of less than 10 nm The carbides are dispersed and precipitated so that the volume ratio with respect to the whole tissue is 0.0007 or more. When this volume ratio is less than 0.0007, even if the average particle diameter of the fine carbide containing Ti is less than 10 nm, since the amount of the fine carbide is small, the desired hot rolled steel sheet strength (tensile strength: 590 MPa or more) can be reliably ensured. It becomes difficult. Therefore, the volume ratio is set to 0.0007 or more. Moreover, when the said volume ratio exceeds 0.004, intensity | strength will become high too much and there exists a possibility that extension flange property may fall. Therefore, it is preferable to make the said volume ratio into 0.0007 or more and 0.004 or less.

In addition, in the present invention, as the precipitation form of the fine carbide containing Ti, in addition to the thermal precipitation which is the main precipitation form, even if randomly deposited fine carbides are mixed, they do not affect the characteristics at all. Regardless of the precipitation form, various precipitation forms will be referred to as dispersion precipitation.

Next, the reason for limitation of the component composition of this invention's hot rolled sheet steel is demonstrated. In addition,% which shows the following component compositions shall mean the mass% unless there is particular notice.

C: 0.005% or more and 0.050% or less

C is an essential element in forming fine carbides and reinforcing a hot rolled steel sheet. When C content is less than 0.005%, the fine carbide of a desired volume ratio cannot be ensured and the tensile strength of 590 Mpa or more cannot be obtained. On the other hand, when C content exceeds 0.050%, while intensity | strength will rise, it will become easy to form pearlite in a steel plate, and it will become difficult to acquire the outstanding elongation flange property. Therefore, C content is made into 0.005% or more and 0.050% or less. Preferably they are 0.020% or more and 0.035% or less. More preferably, they are 0.020% or more and 0.030% or less.

Si: 0.2% or less

Si is a solid solution strengthening element and is an effective element for increasing the strength of steel. However, when Si content exceeds 0.2%, C precipitation from a ferrite phase will be accelerated | stimulated, Fe carbide coarse to a grain boundary will be easy to precipitate, and extension | stretching flange property will fall. In addition, excessive Si adversely affects the plating property. Therefore, Si content is made into 0.2% or less. Preferably it is 0.05% or less.

Mn: 0.8% or less

Since Mn is a solid solution strengthening element and is an element effective for high strength of steel, it is preferable to increase Mn content from a viewpoint of strengthening a hot rolled sheet steel. However, when the Mn content is more than 0.8%, segregation tends to occur, and phases other than the ferrite phase, that is, the hard phase are formed, and the elongation flange property is lowered. Therefore, Mn content is made into 0.8% or less. Preferably it is 0.35% or less. More preferably, it is 0.3% or less.

P: not more than 0.025%

P is a solid solution strengthening element and is an element effective for increasing the strength of steel, but when the P content exceeds 0.025%, segregation becomes remarkable, and elongation flangeability is lowered. Therefore, P content is made into 0.025% or less. Preferably it is 0.02% or less.

S ： 0.01% or less

S is an element that lowers the hot workability (hot rolling property). In addition to increasing the hot cracking sensitivity of the slab, S is present as MnS in the steel and deteriorates the elongation flangeability of the hot rolled steel sheet. Therefore, in this invention, it is preferable to reduce S as much as possible, and you may be 0.01% or less. Preferably it is 0.005% or less.

N: not more than 0.01%

N is a harmful element in this invention, and it is preferable to reduce N as much as possible. In particular, when N content exceeds 0.01%, elongation flange property falls because coarse nitride produces | generates in steel. Therefore, N content is made into 0.01% or less. Preferably it is 0.006% or less.

Al: 0.06% or less

Al is an element which acts as a deoxidizer. In order to acquire such an effect, it is preferable to contain 0.001% or more, but the content exceeding 0.06% reduces elongation and elongation flange property. For this reason, Al content is made into Al: 0.06% or less.

Ti: 0.05% or more and 0.10% or less

Ti is the most important element in this invention. Ti is an element which contributes to the high strength of a steel plate, by forming carbide and maintaining the outstanding elongation flange property. If the Ti content is less than 0.05%, the desired hot rolled steel sheet strength (tensile strength: 590 MPa or more) cannot be secured. On the other hand, when Ti content exceeds 0.10%, there exists a tendency for extending | stretching flange property to fall. Therefore, Ti content is made into 0.05% or more and 0.10% or less. Preferably they are 0.065% or more and 0.095% or less.

The hot rolled sheet steel of this invention contains S, N, Ti so that it may satisfy | fill Formula (1) in said range.

Ti? 0.04 + (N / 14 x 48 + S / 32 x 48)... (One)

(S, N, Ti: content of each element (mass%))

Equation (1) is a very important index in the present invention, which is a requirement to be satisfied in order to bring the fine carbide containing Ti into the desired precipitation state described above.

Ti? 0.04 + (N / 14 x 48 + S / 32 x 48)... (One)

As described above, in the present invention, the fine carbide containing Ti is dispersed and precipitated in the hot rolled steel sheet, but the fine carbide dissolves the carbide in the steel material by heating before hot rolling, and mainly precipitates during winding after hot rolling. Let's do it. In order to deposit the fine carbide stably with an average particle diameter of less than 10 nm and to disperse precipitate so as to have a volume ratio of 0.0007 or more in volume ratio with respect to the entire structure, the amount of Ti serving as the precipitation nuclei of the fine carbide is sufficiently secured. You need to be. In the high temperature region, however, Ti is more likely to form nitrides or sulfides than carbides. Therefore, when Ti content is insufficient with respect to N and S content of steel materials, the amount of Ti used as a precipitation nuclei of fine carbides will decrease with precipitation of the said nitride or sulfide, and the desired volume ratio (0.0007) for the fine carbide containing Ti is desired. It becomes difficult to precipitate by the above).

Therefore, in this invention, Ti, N, and S content are controlled so that (1) Formula Ti≥0.04 + (N / 14 * 48 + S / 32 * 48) is satisfied. Thereby, the amount of Ti which becomes the nucleus of the precipitation of fine carbide is sufficiently secured, and the fine carbide is precipitated stably with an average particle diameter of less than 10 nm, and dispersed and precipitated so that the volume ratio with respect to the whole structure becomes 0.0007 or more. Can be.

In addition, in the present invention, the steel material is heated to the austenite region before hot rolling to dissolve the carbide in the steel material, and the carbide containing Ti is precipitated at the same time as the subsequent austenite to ferrite transformation. However, when the austenite to ferrite transformation temperature is high, carbides containing precipitated Ti become coarse. Therefore, in the present invention, by adjusting the temperature of austenite → ferrite transformation (Ar ₃ transformation point) to the coiling temperature range, it is desirable to deposit upon winding the carbide containing Ti. As a result, the coarsening can be suppressed, and a carbide having an average particle diameter of less than 10 nm can be obtained.

In adjusting the temperature (Ar ₃ transformation point) of the austenite to ferrite transformation to the winding temperature range, in addition to the above-described composition, B: 0.0003% or more and 0.0035% or less so as to satisfy the following formula (2). It is preferable to contain.

B ≧ 0.001 × Mn... (2)

B: 0.0003% or more and 0.0035% or less

B is an element for lowering the Ar ₃ transformation point or lecture, in the present invention, the addition of B by lowering the Ar ₃ transformation point teaching, it is possible to achieve the miniaturization of the carbide containing Ti. The B content is less than 0.0003%, Ar ₃ transformation point is not to be so reduced, but is not sufficiently express the effect of the fine carbide containing Ti. On the other hand, even if it contains exceeding 0.0035%, the said effect is saturated. Therefore, it is preferable to make B content into 0.0003% or more and 0.0035% or less. More preferably, they are 0.0005% or more and 0.0020% or less.

B ≧ 0.001 × Mn... (2)

In this invention, when it contains B, it is also important to control the ratio of B content and Mn content in steel to an appropriate range. MEANS TO SOLVE THE PROBLEM The present inventors examined the means which disperse | distributes and precipitates carbide containing Ti finely (average particle diameter less than 10 nm) in the matrix whose area ratio with respect to the whole ferrite structure is 95% or more. As a result, adjusting the temperature (Ar ₃ transformation point) of the austenite to ferrite transformation in the hot rolling process to the winding temperature range described later makes it extremely fine to reduce the carbide containing Ti to an average particle diameter of less than 10 nm. New knowledge was found to be a valid means.

Further, the inventors of the present invention have further studied, and in the steel composition of the present invention, the B content and the Mn content of the steel material can be controlled to satisfy the desired relationship, whereby the Ar ₃ transformation point of the steel can be adjusted to the target range. I found out. When B is less than (0.001 x Mn), the Ar ₃ transformation point of the steel becomes high, and the carbide containing Ti is not sufficiently refined. Therefore, when it contains B, it is set to B≥0.001xMn.

In addition, in this invention, if content of Mn which is a solid solution strengthening element is more than 0.35%, desired steel plate strength (tensile strength: 590 Mpa or more) can be ensured, without using the above-mentioned B effect. However, when content of Mn is 0.35% or less, it may be difficult to ensure desired steel plate strength, without utilizing the above-mentioned B effect. Therefore, when content of Mn is 0.35% or less, it is preferable to contain B in order to refine | purify the carbide containing Ti further.

Moreover, in this invention, it is preferable to adjust content of C, S, N, Ti so that it may satisfy | fill Formula (4) in said range.

C / 12> Ti / 48-N / 14-S / 32... (4)

(C, S, N, Ti: content of each element (mass%))

As described above, carbides containing Ti tend to be fine carbides having a very small average particle diameter. However, when Ti bonded to C becomes more than C in atomic ratio, carbides tend to coarsen. And with the coarsening of carbide, the precipitation strengthening amount by carbide falls, and it becomes difficult to ensure desired hot-rolled steel sheet strength (tensile strength: 590 Mpa or more).

Therefore, in this invention, it is preferable to define C, Ti, N, S content like (4) Formula. That is, in the present invention, the atomic% (C / 12) of C to the C and Ti contained in the steel material is the atomic% of Ti that can contribute to carbide formation (Ti / 48-N / 14-S / 32 It is preferable to make more than). Thereby, coarsening of the fine carbide containing Ti can be suppressed.

In addition, in this invention, instead of defining C, Ti, N, and S content as Formula (4), the atomic ratio of C and Ti of the fine carbide containing Ti can also be prescribed | regulated as Formula (3).

C / Ti> 1.0... (3)

(C / Ti: Atomic Ratio of C and Ti in Fine Carbide)

In addition, when Ti in a carbide becomes C or more, it becomes easy to make a carbide coarse because it is because the coarsening of a carbide is rate-limited by the diffusion of Ti which is late in diffusion.

In the steel sheet of the present invention, any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, and Ta may be 0.1% or less, preferably 0.03% or less in total. do. In addition, components other than the above are Fe and an unavoidable impurity.

Moreover, the steel plate of this invention may have a plating film on the surface. By forming a plated film on the surface of the steel sheet, the corrosion resistance of the hot rolled steel sheet is improved, and a hot rolled steel sheet which is suitable for a material exposed to a harsh corrosive environment, for example, a suspension part of an automobile, is obtained. Moreover, as a plating film, a hot dip galvanized film, an alloying hot dip galvanized film, etc. are mentioned, for example.

Next, the manufacturing method of the hot rolled sheet steel of this invention is demonstrated.

This invention hot-rolls consisting of rough rolling and finish rolling to a steel raw material, and after completion | finish rolling finishes, it cools and winds up and makes a hot rolled sheet steel. At this time, the finish rolling temperature of finish rolling is set to 880 degreeC or more, the average cooling rate is set to 10 degreeC / s or more, and it is characterized by making winding temperature 550 degreeC or more and less than 800 degreeC.

In this invention, the solvent method of a steel raw material is not specifically limited, Well-known solvent methods, such as a converter and an electric furnace, can be employ | adopted. Moreover, although it is preferable to set it as a slab (steel material) by the continuous casting method from the problem of segregation etc. after a solvent, you may make it a slab by well-known casting methods, such as the ingot-fragment rolling method and the thin slab continuous casting method. In addition, when hot-rolling a slab after casting, you may roll after reheating a slab in a heating furnace, and, if maintaining the temperature more than predetermined temperature, you may roll directly without heating a slab.

Although rough rolling and finish rolling are performed to the steel raw material obtained as mentioned above, in this invention, it is necessary to melt | dissolve the carbide in a steel raw material before rough rolling. In this invention containing Ti which is a carbide formation element, it is preferable to make heating temperature of steel materials into 1150 degreeC or more. However, when the heating temperature of the steel material becomes excessively high, the surface is excessively oxidized, TiO ₂ is generated, and Ti is consumed, so that when the steel sheet is used, a decrease in hardness near the surface tends to occur. It is preferable to set it as 1300 degrees C or less. In addition, as mentioned above, when the steel raw material before rough rolling maintains the temperature more than predetermined temperature, and the carbide in a steel raw material melt | dissolves, the process of heating the steel raw material before rough rolling can be skipped. In addition, it does not need to specifically limit about rough rolling conditions.

Finish rolling temperature: More than 880 degrees Celsius

Optimizing the finish rolling temperature becomes important in securing the elongation and elongation flangeability of the hot rolled steel sheet and reducing the rolling load in the finish rolling. When finish rolling temperature is less than 880 degreeC, the crystal grain of a hot-rolled steel plate surface layer will become a coarse grain, and elongation flange property will be inhibited. Moreover, since rolling is performed in the unrecrystallized temperature range, carbide of Ti coarse precipitates in the old austenite grain boundary, and extension | stretching flange property falls. Therefore, finish rolling temperature shall be 880 degreeC or more. Preferably it is 900 degreeC or more. When the finish rolling temperature is excessively high, grains are coarsened and adversely affect the securing of the desired steel sheet strength (tensile strength: 590 MPa or more). Therefore, the finish rolling temperature is preferably 1000 ° C. or less.

Average cooling rate: More than 10 degrees Celsius / s

After finish rolling, when the average cooling rate is less than 10 ℃ / s to a coiling temperature from above 880 ℃ temperature, the higher the Ar ₃ transformation point, not the carbide containing Ti sufficiently fine. Therefore, the said average cooling rate shall be 10 degrees C / s or more. Preferably it is 30 degreeC / s or more.

Winding temperature: more than 550 ℃ less than 800 ℃

The winding temperature optimization is a fine structure containing the structure of the hot rolled steel sheet over the entire width direction of the hot rolled steel sheet, that is, a matrix having 95% or more of the ferrite phase in an area ratio with respect to the entire structure, and a Ti having an average particle diameter of less than 10 nm. Carbide is dispersed and precipitated, and it is very important for making the structure whose fine carbide is 0.0007 or more by volume ratio with respect to the whole structure | tissue.

If the coiling temperature is less than 550 ° C., the precipitation of fine carbide becomes insufficient at the rolling material width direction edge part which is likely to be in a supercooled state, and it becomes difficult to give a desired steel sheet strength (tensile strength: 590 MPa or more). . Moreover, a problem arises that it becomes difficult to secure the running stability on the runout table. On the other hand, when a coiling temperature becomes 800 degreeC or more, a pearlite will generate | occur | produce and it will become difficult to make a ferrite phase into a matrix which is 95% or more in area ratio with respect to the whole structure. Therefore, it is set as winding temperature 550 degreeC or more and less than 800 degreeC. Preferably they are 550 degreeC or more and less than 700 degreeC, More preferably, they are 580 degreeC or more and less than 700 degreeC.

As described above, in producing a high tensile strength hot rolled steel sheet having excellent workability (elongation flange property) applicable to materials such as suspension parts having a tensile strength (TS) of 590 MPa or more and complex cross-sectional shapes, the average particle size is different. Fine carbides of less than 10 nm need to be dispersed and deposited throughout the entire width direction of the steel sheet in a desired volume ratio (0.0007 or more).

However, in this invention, Ti (Ti≥0.04 + (N / 14 * 48 + S / 32 * 48)) or more of predetermined amount is contained with respect to N and S content in the steel used as a raw material of a hot rolled sheet steel, or further, By containing so that B and Mn content in the steel used as a raw material of a hot rolled sheet steel satisfy | fill a predetermined | prescribed relationship (B≥0.001 * Mn), it is controlled by the composition which the fine carbide of average particle diameter less than 10 nm disperse | distributes sufficiently and precipitates. Therefore, according to the present invention, it is possible to disperse and precipitate fine carbides having an average particle diameter of less than 10 nm at a desired volume ratio (0.0007 or more) over the entire width direction even if the conditions for manufacturing the hot rolled steel sheet are not strictly defined. Uniform and good characteristics (tensile strength, elongation flangeability) are provided over the whole steel plate width direction.

In addition, in this invention, you may form a plating film in the steel plate surface by performing a plating process with respect to the hot rolled steel plate manufactured as mentioned above. For example, an alloying hot dip galvanizing coating may be formed on the surface of the steel sheet by performing a hot dip galvanizing treatment as a plating treatment to form a hot dip galvanizing coating, or further performing an alloying treatment after the hot dip galvanizing treatment.

Example

(Example 1)

The molten steel of the composition shown in Table 1 was solvent-cast continuously by the well-known method, and it was set as the slab (steel material) of thickness 250mm. After heating these slabs at 1250 degreeC, they are rough-rolled, the finishing rolling made into the finishing rolling temperature shown in Table 2 is performed, and after completion | finish rolling is finished, the temperature range from the temperature of 880 degreeC to a coiling temperature is shown in Table 2 It cooled at the average cooling rate, it wound up at the coiling temperature shown in Table 2, and it was set as the hot rolled sheet steel of plate | board thickness: 2.3 mm. In addition, about some hot-rolled steel sheets (hot-rolled number 2 of steel No.A), it is immersed in the zinc plating bath (0.1% Al-Zn) of 480 degreeC, and after forming the hot-dip galvanizing film of 45 g / m <2> of adhesion amounts, And alloying treatment at 520 ° C. to obtain an alloyed hot dip galvanized steel sheet.

A test piece was taken from the hot-rolled steel sheet obtained above, a structure observation, a tensile test, a hole expansion test were carried out, and the area ratio of ferrite phase, the average particle diameter and volume ratio of the fine carbide containing Ti, and C in the fine carbide The atomic ratio, tensile strength, and hole expansion ratio (elongation flange property) of Ti were calculated | required. The test method was as follows.

(i) tissue observation

The test piece was taken from the obtained hot-rolled steel sheet, and the cross section parallel to the rolling direction of the test piece was mechanically polished and corroded with a nital, and then magnified by a scanning electron microscope (SEM): tissue photograph taken at 3000 times (SEM photograph). Using the image analysis apparatus, the types of structures other than the ferrite phase, the ferrite phase, and the area ratio thereof were determined.

Moreover, the thin film manufactured from the hot-rolled steel sheet was observed by the transmission electron microscope (TEM) at the magnification of 120000-260000 times, and the particle diameter and volume ratio of the fine carbide containing Ti were calculated | required.

Moreover, the atomic ratio of C and Ti in fine carbide was calculated | required by the energy dispersive X-ray spectroscopy apparatus (EDX) attached to TEM.

In addition, the particle size of the fine carbide containing Ti obtained the individual particle diameters by the image processing using the circular approximation based on the observation result of 30 visual field in 260000 times, and made the arithmetic mean of the obtained particle diameters into the average particle diameter. Moreover, from the same observation result which calculated | required the said particle diameter, the area ratio of the fine carbide with respect to the whole structure was calculated | required by image analysis, and this area ratio was made into volume ratio.

(ii) tensile test

From the obtained hot-rolled steel sheet, JIS No. 5 tensile test piece (JIS Z 2201) having a direction perpendicular to the rolling direction as the tensile direction was taken, a tensile test in accordance with JIS Z 2241 was performed, and tensile strength (TS) was obtained. Measured.

(iii) hole expansion test

From a hot-rolled steel sheet thus obtained, test pieces (size: 130 ㎜ × 130 ㎜) collected, and the initial diameter d ₀ on the test piece was formed in the hole 10 in ㎜φ stamping. The hole expansion test was done using these test pieces. That is, a cone punch with a vertex angle of 60 ° is inserted into the hole, the hole is enlarged, and the diameter d of the hole when the crack penetrates through the hot-rolled steel sheet (test piece) is measured. λ (%) was calculated.

Hole expansion rate λ (%) = d (d-d ₀ ) / d ₀ d × 100

The obtained results are shown in Table 3.

All of the examples of the present invention are hot rolled steel sheets having high tensile strength TS: 590 MPa or more and excellent elongation flangeability of hole expansion ratio?: 100% or more. On the other hand, in the comparative example outside the scope of the present invention, the predetermined high strength is not secured or the hole expansion ratio λ is not secured.

(Example 2)

The molten steel of the composition shown in Table 4 was solvent-cast continuously by the well-known method, and it was set as the slab (steel material) of thickness 250mm. These slabs are rough-rolled after heating to 1250 degreeC, finish-rolling made into the finishing rolling temperature shown in Table 5, and after finishing rolling, the average temperature range from the temperature of 880 degreeC to a coiling temperature is shown in Table 5 It cooled at the cooling rate, it wound up at the coiling temperature shown in Table 5, and it was set as the hot rolled sheet steel of plate | board thickness: 2.3 mm. In addition, about some hot-rolled steel sheets (hot rolled number 13 of steel No.H, hot rolled number 15 of steel No.i), it is immersed in the zinc plating bath (0.1% Al-Zn) of 480 degreeC, and adhesion amount 45g / m <2>. Hot dip galvanized film was formed to obtain a hot dip galvanized steel sheet. Moreover, about some hot-rolled steel sheets (hot-rolled number 18 of steel No. J, 22 of steel No. K), after forming a hot-dip galvanized film similarly to the above, alloying process is performed at 520 degreeC, and alloying is carried out. A hot dip galvanized steel sheet was used.

A test piece was taken from the hot-rolled steel sheet obtained as described above, and the structure observation, the tensile test, and the hole expansion test were conducted in the same manner as in Example 1, and the area ratio of the ferrite phase, the average particle diameter and volume ratio of the fine carbide containing Ti, and The atomic ratio, tensile strength, and hole expansion ratio (elongation flange property) of C and Ti in the fine carbide were determined. The obtained results are shown in Table 6.

All of the examples of the present invention are hot rolled steel sheets having high tensile strength TS: 590 MPa or more and excellent elongation flangeability of hole expansion ratio?: 100% or more. On the other hand, in the comparative example outside the scope of the present invention, the predetermined high strength is not secured or the hole expansion ratio λ is not secured.

Claims (10)

In mass%,
C: 0.005% or more, 0.050% or less, Si: 0.2% or less,
Mn: 0.8% or less, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less
Containing S, N, and Ti so as to satisfy the following formula (1), the balance comprising Fe and inevitable impurities, a matrix having an area ratio of 95% or more with respect to the entire ferrite structure, and Ti A fine carbide having an average particle diameter of less than 10 nm is dispersed and precipitated, and has a structure having a volume ratio of 0.0007 or more to the entire structure of the fine carbide, and has a tensile strength of 590 MPa or more.
Ti? 0.04 + (N / 14 x 48 + S / 32 x 48)... (One)
(S, N, Ti: content of each element (mass%)) The method of claim 1,
In addition to the said composition, B: 0.0003% or more and 0.0035% or less are contained by mass% so that following (2) Formula may be satisfied.
B ≧ 0.001 × Mn... (2)
(Mn, B: content of each element (mass%)) 3. The method according to claim 1 or 2,
The said composition satisfy | fills following formula (4), The high tensile strength hot rolled sheet steel excellent in workability.
C / 12> Ti / 48-N / 14-S / 32... (4)
(C, S, N, Ti: content of each element (mass%)) The method according to any one of claims 1 to 3,
Said fine carbide is a carbide which satisfy | fills following formula (3) by the atomic ratio of C and Ti contained, The high tensile strength hot rolled sheet steel excellent in workability characterized by the above-mentioned.
C / Ti> 1.0... (3)
(C / Ti: Atomic Ratio of C and Ti in Fine Carbide) The method according to any one of claims 1 to 4,
In addition to the above-mentioned composition, 0.1% or less of a total of any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, and Ta is further contained. High tensile hot rolled steel sheet with excellent workability. 6. The method according to any one of claims 1 to 5,
High tensile hot rolled steel sheet excellent in workability, which has a plating film on the steel plate surface. When the steel material is subjected to hot rolling consisting of rough rolling and finish rolling, and after finishing rolling, it is cooled and wound up to form a hot rolled steel sheet.
The steel material in mass%,
C: 0.005% or more, 0.050% or less, Si: 0.2% or less,
Mn: 0.8% or less, P: 0.025% or less,
S: 0.01% or less, N: 0.01% or less,
Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less
Is contained so that S, N, and Ti satisfy the following formula (1), and the balance is made of Fe and unavoidable impurities.
The finish rolling temperature of the said finish rolling is 880 degreeC or more, the average cooling rate of the said cooling is 10 degreeC / s or more, and the winding temperature is 550 degreeC or more and less than 800 degreeC, The tensile strength is 590 Mpa The manufacturing method of the high tension hot rolled sheet steel excellent in the above workability.
Ti? 0.04 + (N / 14 x 48 + S / 32 x 48)... (One)
(S, N, Ti: content of each element (mass%)) The method of claim 7, wherein
In addition to the said composition, B: 0.0003% or more and 0.0035% or less are contained by mass% so that following (2) Formula may be satisfied.
B ≧ 0.001 × Mn... (2)
(Mn, B: content of each element (mass%)) 9. The method according to claim 7 or 8,
Said composition satisfy | fills following formula (4), The manufacturing method of the high tension hot rolled sheet steel excellent in workability.
C / 12> Ti / 48-N / 14-S / 32... (4)
(C, S, N, Ti: content of each element (mass%)) 10. The method according to any one of claims 7 to 9,
In addition to the above-mentioned composition, 0.1% or less of a total of any one or more of Cu, Sn, Ni, Ca, Mg, Co, As, Cr, W, Nb, Pb, and Ta is further contained. The manufacturing method of the high tension hot rolled steel sheet excellent in workability.